The strftime() function formats the broken-down time tm according to
the format specification format and places the result in the
character array s of size max. The broken-down time structure tm is
defined in <time.h>. See also ctime(3).
The format specification is a null-terminated string and may contain
special character sequences called conversion specifications, each of
which is introduced by a '%' character and terminated by some other
character known as a conversion specifier character. All other
character sequences are ordinary character sequences.
The characters of ordinary character sequences (including the null
byte) are copied verbatim from format to s. However, the characters
of conversion specifications are replaced as shown in the list below.
In this list, the field(s) employed from the tm structure are also
shown.
%a The abbreviated name of the day of the week according to the
current locale. (Calculated from tm_wday.)
%A The full name of the day of the week according to the current
locale. (Calculated from tm_wday.)
%b The abbreviated month name according to the current locale.
(Calculated from tm_mon.)
%B The full month name according to the current locale.
(Calculated from tm_mon.)
%c The preferred date and time representation for the current
locale.
%C The century number (year/100) as a 2-digit integer. (SU)
(Calculated from tm_year.)
%d The day of the month as a decimal number (range 01 to 31).
(Calculated from tm_mday.)
%D Equivalent to %m/%d/%y. (Yecch—for Americans only. Americans
should note that in other countries %d/%m/%y is rather common.
This means that in international context this format is
ambiguous and should not be used.) (SU)
%e Like %d, the day of the month as a decimal number, but a
leading zero is replaced by a space. (SU) (Calculated from
tm_mday.)
%E Modifier: use alternative format, see below. (SU)
%F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
%G The ISO 8601 week-based year (see NOTES) with century as a
decimal number. The 4-digit year corresponding to the ISO
week number (see %V). This has the same format and value as
%Y, except that if the ISO week number belongs to the previous
or next year, that year is used instead. (TZ) (Calculated from
tm_year, tm_yday, and tm_wday.)
%g Like %G, but without century, that is, with a 2-digit year
(00–99). (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)
%h Equivalent to %b. (SU)
%H The hour as a decimal number using a 24-hour clock (range 00
to 23). (Calculated from tm_hour.)
%I The hour as a decimal number using a 12-hour clock (range 01
to 12). (Calculated from tm_hour.)
%j The day of the year as a decimal number (range 001 to 366).
(Calculated from tm_yday.)
%k The hour (24-hour clock) as a decimal number (range 0 to 23);
single digits are preceded by a blank. (See also %H.)
(Calculated from tm_hour.) (TZ)
%l The hour (12-hour clock) as a decimal number (range 1 to 12);
single digits are preceded by a blank. (See also %I.)
(Calculated from tm_hour.) (TZ)
%m The month as a decimal number (range 01 to 12). (Calculated
from tm_mon.)
%M The minute as a decimal number (range 00 to 59). (Calculated
from tm_min.)
%n A newline character. (SU)
%O Modifier: use alternative format, see below. (SU)
%p Either "AM" or "PM" according to the given time value, or the
corresponding strings for the current locale. Noon is treated
as "PM" and midnight as "AM". (Calculated from tm_hour.)
%P Like %p but in lowercase: "am" or "pm" or a corresponding
string for the current locale. (Calculated from tm_hour.)
(GNU)
%r The time in a.m. or p.m. notation. In the POSIX locale this
is equivalent to %I:%M:%S %p. (SU)
%R The time in 24-hour notation (%H:%M). (SU) For a version
including the seconds, see %T below.
%s The number of seconds since the Epoch, 1970-01-01 00:00:00
+0000 (UTC). (TZ) (Calculated from mktime(tm).)
%S The second as a decimal number (range 00 to 60). (The range
is up to 60 to allow for occasional leap seconds.)
(Calculated from tm_sec.)
%t A tab character. (SU)
%T The time in 24-hour notation (%H:%M:%S). (SU)
%u The day of the week as a decimal, range 1 to 7, Monday being
1. See also %w. (Calculated from tm_wday.) (SU)
%U The week number of the current year as a decimal number, range
00 to 53, starting with the first Sunday as the first day of
week 01. See also %V and %W. (Calculated from tm_yday and
tm_wday.)
%V The ISO 8601 week number (see NOTES) of the current year as a
decimal number, range 01 to 53, where week 1 is the first week
that has at least 4 days in the new year. See also %U and %W.
(Calculated from tm_year, tm_yday, and tm_wday.) (SU)
%w The day of the week as a decimal, range 0 to 6, Sunday being
0. See also %u. (Calculated from tm_wday.)
%W The week number of the current year as a decimal number, range
00 to 53, starting with the first Monday as the first day of
week 01. (Calculated from tm_yday and tm_wday.)
%x The preferred date representation for the current locale
without the time.
%X The preferred time representation for the current locale
without the date.
%y The year as a decimal number without a century (range 00 to
99). (Calculated from tm_year)
%Y The year as a decimal number including the century.
(Calculated from tm_year)
%z The +hhmm or -hhmm numeric timezone (that is, the hour and
minute offset from UTC). (SU)
%Z The timezone name or abbreviation.
%+ The date and time in date(1) format. (TZ) (Not supported in
glibc2.)
%% A literal '%' character.
Some conversion specifications can be modified by preceding the
conversion specifier character by the E or O modifier to indicate
that an alternative format should be used. If the alternative format
or specification does not exist for the current locale, the behavior
will be as if the unmodified conversion specification were used. (SU)
The Single UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY,
%Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy,
where the effect of the O modifier is to use alternative numeric
symbols (say, roman numerals), and that of the E modifier is to use a
locale-dependent alternative representation.

Provided that the result string, including the terminating null byte,
does not exceed max bytes, strftime() returns the number of bytes
(excluding the terminating null byte) placed in the array s. If the
length of the result string (including the terminating null byte)
would exceed max bytes, then strftime() returns 0, and the contents
of the array are undefined.
Note that the return value 0 does not necessarily indicate an error.
For example, in many locales %p yields an empty string. An empty
format string will likewise yield an empty string.

SVr4, C89, C99. There are strict inclusions between the set of
conversions given in ANSI C (unmarked), those given in the Single
UNIX Specification (marked SU), those given in Olson's timezone
package (marked TZ), and those given in glibc (marked GNU), except
that %+ is not supported in glibc2. On the other hand glibc2 has
several more extensions. POSIX.1 only refers to ANSI C; POSIX.2
describes under date(1) several extensions that could apply to
strftime() as well. The %F conversion is in C99 and POSIX.1-2001.
In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for
the theoretical possibility of a minute that included a double leap
second (there never has been such a minute).

ISO 8601 week dates%G, %g, and %V yield values calculated from the week-based year
defined by the ISO 8601 standard. In this system, weeks start on a
Monday, and are numbered from 01, for the first week, up to 52 or 53,
for the last week. Week 1 is the first week where four or more days
fall within the new year (or, synonymously, week 01 is: the first
week of the year that contains a Thursday; or, the week that has 4
January in it). When three of fewer days of the first calendar week
of the new year fall within that year, then the ISO 8601 week-based
system counts those days as part of week 53 of the preceding year.
For example, 1 January 2010 is a Friday, meaning that just three days
of that calendar week fall in 2010. Thus, the ISO 8601 week-based
system considers these days to be part of week 53 (%V) of the year
2009 (%G); week 01 of ISO 8601 year 2010 starts on Monday, 4 January
2010.
Glibc notes
Glibc provides some extensions for conversion specifications. (These
extensions are not specified in POSIX.1-2001, but a few other systems
provide similar features.) Between the '%' character and the
conversion specifier character, an optional flag and field width may
be specified. (These precede the E or O modifiers, if present.)
The following flag characters are permitted:
_ (underscore) Pad a numeric result string with spaces.
- (dash) Do not pad a numeric result string.
0 Pad a numeric result string with zeros even if the conversion
specifier character uses space-padding by default.
^ Convert alphabetic characters in result string to uppercase.
# Swap the case of the result string. (This flag works only
with certain conversion specifier characters, and of these, it
is only really useful with %Z.)
An optional decimal width specifier may follow the (possibly absent)
flag. If the natural size of the field is smaller than this width,
then the result string is padded (on the left) to the specified
width.

If the output string would exceed max bytes, errno is not set. This
makes it impossible to distinguish this error case from cases where
the format string legitimately produces a zero-length output string.
POSIX.1-2001 does not specify any errno settings for strftime().
Some buggy versions of gcc(1) complain about the use of %c: warning:`%c' yields only last 2 digits of year in some locales. Of course
programmers are encouraged to use %c, it gives the preferred date and
time representation. One meets all kinds of strange obfuscations to
circumvent this gcc(1) problem. A relatively clean one is to add an
intermediate function
size_t
my_strftime(char *s, size_t max, const char *fmt,
const struct tm *tm)
{
return strftime(s, max, fmt, tm);
}
Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the
warning, so that the above workaround is no longer required.

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 STRFTIME(3)